Field of the Invention
The present invention relates generally to a system for controlling the flow rate of intake air in an internal combustion engine for a motor vehicle by controlling electrically a throttle valve. More particularly, the invention is concerned with an apparatus for fail-safing the intake air flow control system.
Description of the Related Art
The intake air flow fed to a gasoline engine is conventionally regulated by a throttle valve whose opening is controlled by means of an accelerator pedal mechanically interlocked with the valve. In recent years, however, there is adopted in some practical applications a so-called wired intake air flow control system in which the opening of the throttle valve is controlled by an electrical actuator in response to an output signal of an accelerator pedal position sensor which signal represents depth of depression of an accelerator pedal of a motor vehicle, in an effort to enhance the comfortableness in driving the motor vehicle and realize a driving at a cruising speed, while improving a disposition layout of engine accessories.
For a better understanding of the present invention, the background technique thereof will be described in some detail.
FIG. 2 shows generally and schematically a structure of an intake air flow control system for an internal combustion engine (hereinafter also referred to simply as the engine) of a motor vehicle known heretofore. Referring to the figure, the engine denoted by a reference numeral 1 is equipped with an intake pipe 14 in which a throttle valve 2 is installed for controlling or adjusting the amount or quantity of air supplied to the engine. The throttle valve 2 is mechanically coupled to an electrical throttle actuator 3 which may be constituted by a DC motor, a stepping motor or the like for actuating the throttle valve 2. To this end, the throttle valve 2 is connected to the throttle actuator 3 by means of a shaft 4. A return spring 5 is wound around the shaft 4 in such disposition that the throttle valve 2 is resiliently urged toward the closed position when operation of the throttle actuator 3 is disabled. Provided in association with the throttle valve 2 is a throttle position sensor (TPS) 6 for detecting the degree of opening of the throttle valve 2. On the other hand, there is provided in association with an accelerator pedal 7 of a motor vehicle an accelerator pedal position sensor (APS) 8 which serves for detecting the degree or depth of depression (actuation level) of the accelerator pedal 7. A reference numeral 9 denotes an engine rotation speed sensor for detecting the rotation speed (rpm) of the engine 1 to thereby generate an engine rotation speed signal. The output signals of the sensors 6, 8 and 9 mentioned above are inputted to a controller 10 which is designed to control operation of the throttle actuator 3 on the basis of the sensor output signals.
With the intake air flow control system of the structure described above, the control of the throttle actuator 3 is realized through a procedure which is illustrated in a flow chart of FIG. 3. Incidentally, processings and operations such as arithmetic operations, conditional decisions and others for the aimed control described below by reference to FIG. 3 and others are executed by a micro-computer incorporated in the controller 10. However, since such micro-computer is a conventional one, description thereof is omitted, being understood that implementation as well as programming of the micro-computer lies within the skill of those having ordinary knowledge in the art.
Now, referring to FIG. 3, in a step 100, the controller 10 or micro-computer incorporated therein fetches the output signal of the accelerator pedal position sensor 8, which signal represents a degree of depression .alpha. of the accelerator pedal 7. In a step 101, a desired opening degree .theta..sub.s of the throttle valve 2 is arithmetically determined on the basis of the detected depression depth .alpha. of the accelerator pedal 7 and the engine speed (rpm) Ne indicated by the output signal of the engine speed sensor 9. The arithmetic operation for determining the throttle opening .theta..sub.s can be performed in accordance with a predetermined function which represents a relation between the desired throttle opening .theta..sub.s and the accelerator pedal depression a by taking into consideration the engine speed Ne as a correcting quantity.
The relation between the desired throttle valve opening degree .theta..sub.s and the accelerator pedal depression depth .alpha. may differ in dependence on maneuvering performances or characteristics to be imparted to the motor vehicle. FIG. 4 graphically illustrates a typical one of such relations. Referring to the figure, a characteristic curve a in solid line indicates that the throttle opening degree .theta..sub.s is changed substantially linearly in proportion to the depression .alpha. of the accelerator pedal. In contrast, in the case represented by a characteristic curve b, the throttle valve opening .theta..sub.s is so controlled as to change gently in a range within which the accelerator pedal depression .alpha. remains small. With the maneuvering characteristic represented by the curve b, it is contemplated to cope with such problems that shock is likely to occur in the motor vehicle, making difficult the optimum control of the engine, when the intake air flow changes rapidly or steeply upon starting of the vehicle or in the course of running at a low speed.
On the other hand, FIG. 5 graphically illustrates relations between the engine output torque and the engine rotation speed (rpm). As can be seen from a broken-line curve labeled "ENGINE TORQUE", the output torque of the engine does not bear a linear relationship to the engine rotation speed (rpm). More specifically, in low- and high-speed ranges, the engine output torque tends to become low. In this conjunction, it is to be mentioned that the above-mentioned dependence of the engine output torque on the engine rotation speed ranges can be improved by correcting the relation represented by the curve b shown in FIG. 4 with correcting coefficients or quantities represented by a solid-line curve shown in FIG. 5 and labeled "CORRECTING COEFFICIENTS". At this juncture, it should also be mentioned that the control characteristic of the throttle opening degree .theta..sub.s relative to the accelerator pedal depression .alpha. described above is only for the purpose of illustration. In reality, such control characteristics may vary in dependence on the desired maneuverability, comfortableness in driving the motor vehicle and/or other factors as well as performances of the engine.
After having determined the desired throttle opening degree .theta..sub.s as mentioned above, the processing proceeds to a step 102 (FIG. 3) in which a real or actual throttle opening degree .theta..sub.r is fetched from the output of the throttle position sensor 6, which is then followed by a step 103 where a deviation or difference e between the desired throttle opening degree .theta..sub.s and the real throttle opening degree .theta..sub.r is arithmetically determined. When the real throttle opening degree .theta..sub.r is smaller than the desired throttle opening .theta..sub.s, the throttle valve 2 is driven in the direction to increase the throttle opening .theta..sub.r on the basis of the deviation e through the throttle actuator 3 in a step 104a. If otherwise, the throttle valve 2 is driven in the direction to decrease the throttle opening .theta..sub.r through the throttle actuator 3 in a step 104b.
By controlling the opening degree of the throttle valve 2 through the electrical throttle actuator 3 in this manner, a high controllability of the engine operation and hence a high maneuverability of the motor vehicle can be realized. However, in contrast to the conventional mechanical control of the throttle valve in which the opening degree thereof is controlled by the accelerator pedal through the medium of a mechanical linkage, the electrical control of the throttle valve 2 described above is susceptible to a problem that the throttle valve 2 may become inoperative, when a failure occurs in the throttle actuator 3, the controller 10 or other components taking parts in the electrical control of the throttle valve 2, as a result of which uncontrollable running or runaway of the motor vehicle will be incurred. Accordingly, it is very important to fail-safe the electrical control of the throttle valve mentioned above.
FIG. 6 is a flow chart for illustrating, by way of example, a procedure known heretofore for making decision as to occurrence of abnormality in the control system for the throttle valve 2 inclusive of the throttle actuator 3 together with the measures to be taken in dependence on the results of the abnormality decision. Referring to the figure, in a step 200, difference .beta. between the desired opening degree .theta..sub.s of the throttle valve which can be derived on the basis of the depression .alpha. of the accelerator pedal 7 and the real opening degree .theta..sub.r of the throttle valve is determined. In this conjunction, it is noted that the relation between the quantities .alpha. and .theta..sub.r can be given by a predetermined function, as described above. So long as the relation given by this function remains normal, there can not make appearance the difference .beta. of such magnitude which exceeds a predetermined value .beta..sub.1. In other words, it can be decided that the real throttle opening .theta..sub.r is abnormal when the above-mentioned difference .beta. exceeds the predetermined value .beta..sub.1. When it is determined in a step 201 that the difference .beta. is greater than the preset value .beta..sub.1, electric power supply to the throttle actuator 3 is interrupted to thereby stop the operation of the throttle actuator 3 in a step 202b, because, if otherwise, there arises possibility of runaway of the motor vehicle due to the abnormality occurring in the throttle valve or in the control system therefor. On the other hand, when the difference .beta. is smaller than the preset value .beta..sub.1, this means that the throttle actuator 3 is controlled normally in a step 202a.
When operation of the throttle actuator 3 is disabled, the throttle valve 2 is resiliently urged to move to the fully closed position under the effort of the return spring 5. However, there may arise such situation that the throttle valve 2 can not be moved to the fully closed position even when the operation of the throttle actuator 3 is stopped, because of a frictional engagement in a reduction gear train incorporated in the throttle actuator 3. To deal with this problem, it is known to dispose an electromagnetic clutch (not shown) between the shaft 4 of the throttle valve 2 and the throttle actuator 3 and disconnect the former from the latter by deenergizing the electromagnetic clutch upon occurrence of abnormality in the throttle valve control system to thereby allow the throttle valve 2 to assume the fully closed position under the action of the return spring 5. Further, when a failure takes place in operation for opening the throttle valve 2, fuel injection to all or some of the engine cylinders may be interrupted to thereby lower the engine output torque.
On the other hand, when a failure occurs in the fully closed state of the throttle valve 2 (i.e., when the throttle valve 2 can not be opened from the fully closed position), the engine operation stops spontaneously to inhibit the motor vehicle from further running. As the measures for coping with this problem, it can be conceived to mechanically couple the throttle valve 2 to the accelerator pedal 7 through manual operation or to provide throttle actuator systems in duplicate with one in redundancy.
As is apparent from the foregoing description, in the conventional throttle valve control system for the motor vehicle, the throttle actuator control can certainly be failsafed against failure in the opened position of the throttle valve (i.e., failure incurring runaway of the engine). However, for the failure occurring in the fully closed position of the throttle valve (i.e., failure disabling the running of the engine), there is known no remedy method which can be adopted practically and profitably from the standpoint of manipulation as well as the economical viewpoint. It goes without saying that the disability of running the motor vehicle due to such failure will incur unwanted situation in dependence on the place where the motor vehicle is driven. For example, in the course of driving the motor vehicle on an expressway, this sort of failure should positively be excluded.